This invention relates to thermoplastic resin compositions and more particularly is concerned with polycarbonate resin mixtures having improved impact strength, especially in thick sections, and extraordinary resistance to environmental stress crazing and cracking.
Aromatic carbonate polymers are well known, commercially available materials having a variety of applications in the plastics art. Such carbonate polymers may be prepared by reacting a dihydric phenol, such as 2,2-bis(4-hydroxyphenyl)-propane, with a carbonate precursor, such as phosgene, in the presence of an acid binding agent. Generally speaking, aromatic polycarbonate resins offer a high resistance to the attack of mineral acids, may be readily molded, and are physiologically harmless as well as strain resistant. In addition, such polymers have a high tensile and impact strength (except in thick molded sections), and a dimensional stability surpassing that of other thermoplastic materials. However, in certain applications, the use of aromatic polycarbonate resins is limited because they exhibit severe environmental stress crazing and cracking. "Environmental stress crazing and cracking" refers to the type of failure which is hastened by the presence of organic solvents such as, for example, gasoline, particularly high octane, no lead gasoline, acetone, heptane and carbon tetrachloride when such solvents are in contact with stressed parts fabricated from aromatic polycarbonate resins. The most significant effect is a loss in vital impact strength and also an increase in brittle-type failure. Contact with such solvents may occur, for example, when parts are used under the hood of automobiles, or near the gasoline filler ports thereof, or when solvents are used to clean or degrease stressed parts made from polycarbonate resins.
At present, no entirely satisfactory means is available for reducing environmental stress crazing and cracking of polycarbonate resins, although a variety of methods have been proposed.
In Goldblum, U.S. Pat. No. 3,431,224, assigned to the same assignee as this application, for example, it is proposed to add modifiers to polycarbonate, in certain proportions, the modifiers comprising at least one member of the class consisting of polyethylene, polypropylene, polyisobutylene, a copolymer of ethylene and an ethyl acrylate, a copolymer of ethylene and propylene, a cellulose ester, a polyamide, a polyvinyl acetal, an alkyl cellulose ether, and a polyurethane elastomer. While the results with such modifiers are generally excellent, in thin sections, e.g., 1/8 inch, it has been found that there is a tendency for failure to occur with these modifiers in thicker molded parts, e.g., of 1/4 inch thickness, and such failure is of the undesirable brittle type, especially after exposure to high test gasoline. Another modifier proposed to be added to polycarbonate is reported in Research Disclosure No. 20810. Dow Chemical Company, August, 1981. Data are provided showing that polycarbonate modified with a linear low density polyolefin, namely ethylene/octene-1 copolymer, provide good impact strength at increased part thickness. There is no suggestion therein that such a modifier will significantly enhance resistance to environmental stress crazing and cracking, and, as will be shown hereinafter, soaking a composition modified with a linear low density copolymer of ethylene and octene-1, even in thin sections, causes the impact strength to deteriorate substantially and results in brittle failure. Still other modifiers have been proposed for impact strength improvement, but none of them provides optimum environmental stress crazing and cracking resistance--applicant's earlier filed commonly assigned U.S. patent applications, Ser. No. 238,643, filed Feb. 26, 1981, now abandoned; Ser. No. 343,949, filed Feb. 29, 1982, now U.S. Pat. No. 4,430,476; Ser. No. 352,382, filed Feb. 25, 1982, now U.S. Pat. No. 4,444,949, and Ser. No. 421,788, filed Sept. 23, 1982, being expressly mentioned in this connection. Ser. Nos. 238,643 and 352,382 describe polycarbonates modified with a combination of a butadiene-styrene block copolymer of the coupled resinous type, an acrylate core-shell interpolymer and, optionally, an olefin/acrylate copolymer. Such compositions process well and are toughened, but there is no disclosure of significant solvent resistance and, as will be shown later herein, by themselves, the coupled resinous block copolymers do not provide significant resistance to environmental stress crazing and cracking at relatively low and moderate blending levels, even in thin sections. Ser. No. 343,959, now abandoned, describes polycarbonate resins modified with a combination of the coupled resinous block copolymers and a linear low density polyolefin resin. There is no mention that such modifier combiantions will provide enhanced resistance to environmental stress crazing and cracking. Ser. No. 421,788 discloses that polycarbonate resins modified with coupled resinous block copolymers, olefin/acrylate copolymers and, optionally, polyolefins have good environmental stress crazing and cracking resistance.